Implement and parse the admonition node

[stsewd]

We have a Note node

lira/lira/parsers/nodes.py

Lines 137 to 143 in f05975a

	class Note(Node):
	valid_options = {"title"}
	def __str__(self):
	title = self.options["title"]
	return f"{self.tagname} ({title}): {self.children}"

but we actually want an Admonition node with different types (note, warning, tip).

[stsewd]

Or maybe we are fine having a different class?

[stsewd]

This is implemented, we need to parse it now.